1. Field of the Invention
The present invention relates to an image forming apparatus and a brush member used in the image forming apparatus. More particularly, the present invention relates to an image forming apparatus (i.e., a copier, facsimile machine, printer and so forth) using a brush member applying a transfer bias with respect to the back side of an endless belt member to transfer an image on the surface of the endless belt member or on a recording medium carried by the endless belt member.
2. Discussion of the Related Art
A conventional background image forming apparatus includes a transfer device having a transfer brush serving as a brush member. The transfer brush includes a conductive brushing fiber portion formed by a plurality of fibers or yarns that are arranged in a standing condition on the surface of a metallic supporting plate, and is usually applied with a predetermined electrical resistance.
FIGS. 1 and 2 show examples of defects in an image caused by the conventional background image forming apparatuses due to the above-described background transfer brush.
A plurality of raised fibers are formed by a conductive material, for example, rayon, nylon or so forth. The transfer brush using the above-described conductive material have easily caused black streaks in a belt moving direction on the halftone area of an output image, as shown in FIG. 1. The black streaks in the belt moving direction are formed along a direction to which a belt member moves or travels, as indicated by arrow E in FIG. 1. The patterns of the black streaks are formed as though the image has been scratched by the plurality of fibers of the transfer brush.
In addition, the inventor of the present invention has found that conventional background image forming apparatuses, using the above-described transfer brush, tend to easily cause sharp changes in image density on the halftone area of an output image. FIG. 2 shows an example of the sharp changes in image density. This is a phenomenon where the density in the halftone image of an output image is rapidly increased after a recording medium, having the output image thereon, has passed a predetermined position in the direction to which the belt member travels, as indicated by arrow E in FIG. 2. The rapid change shown in the enlarged image in FIG. 2 is observed on the area closer to the trailing edge of a recording medium than the area closer to the leading edge thereof. Specifically, when a drum-shaped photoconductive element having a diameter of 100 mm is used as an image bearing member, for example, the rapid changes in image density are observed at a location approximately 314 mm from the leading edge of an A3 size paper.
The inventor of the present invention has found the cause of the above-described black streaks. The cause of the black streaks is described below.
The plurality of raised fibers that form the brushing fiber portion of a transfer brush have deviations in respective electric resistance values. With a relatively small amount of deviations in respective electric resistance values of the plurality of raised fibers, the transfer device applies electric current in a substantially uniform manner from each single fiber thereof to the belt member.
On the other hand, when the transfer brush has a relatively large amount of deviations in respective electric resistance values of the plurality of raised fibers, a greater amount of electric current flows to the belt member from the fibers thereof having a small electric resistance value than the fibers thereof having a large electric resistance value. Under the above-described conditions, the excess electric current is applied from the raised fibers having a small electric resistance value and flows to an image bearing member via the belt member. The flow of the excess electric current can cause a reverse charging on the surface of the image bearing member, resulting in a formation of patterns having streaks. The reversely charged area with streaks on the surface of the image bearing member cannot be sufficiently discharged by a discharging lamp. When the surface of the image bearing member is uniformly charged again after the discharging operation by the discharge lamp, the previously reversely charged area on the image bearing member may have a potential substantially lower than the other areas. Further, a greater amount of development material, such as toner, may adhere on the previously reversely charged area than the other areas. Thereby, the black streaks are shown in the sheet traveling direction.
The inventor of the present invention has also found that the above-described sharp changes in image density can be found at a position to which an image is transferred from an image bearing member. The sharp changes are caused by a timing of when the image bearing member is rotated by one cycle since the start of application of the transfer bias to the transfer brush. The occurrence of the sharp changes in image density is caused by the insufficient discharging of the excess electric current. The excess electric current may be leaked from the transfer brush via the belt member to the image bearing member, but the discharge lamp cannot remove the excess electric current sufficiently. Hence, the potential on the surface of the image bearing member may be lower than usual after the second rotation of the image bearing member.
Further, the inventor of the present invention has found that a transfer brush having the brushing fiber portion with a relatively low electric resistance value tends to easily cause the sharp changes in image density.